[OpenGL] Issue when passing gl_FrontColor through Tessellation & Geometry shader stages

[vsarthou]

Hello,

I am using OpenGL 4.6 in Compatibility Profile mode on an HD Graphics 630, and I am encountering a rendering issue when using the gl_FrontColor built-in shader variable through Tessellation and Geometry shader stages.

It seems that the value of gl_FrontColor incorrectly becomes (0,0,0,0) at some point through the stages.

I am not experiencing this issue on Nvidia or AMD GPUs.

I made a minimal reproducer that I attached in 3 versions: the executable, the source code, and an OpenGL trace generated using apitrace (https://github.com/apitrace/apitrace).

The reproducer renders a full-window triangle, and sets a different gl_FrontColor value for each vertex.

The expected rendering can be seen in OK.png.

The rendering I get is just an all-black frame buffer (KO.png).

Other info:

• The issue is still reproducible if I only use a Geometry shader (Vert => Geom => Frag)
• The issue is still reproducible if I only use Tessallation shaders (Vert => TessCtrl => TessEval => Frag)
• There is no issue if I don't use Tessellation or Geometry shader stages (only Vertex + Fragment).

More detailed info about my setup:

System Setup Information:
-----------------------------------------

CPU: Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
GPU: Intel® UHD Graphics 630
Graphics Driver Version: 31.0.101.2115
Operating System: Windows 10 Enterprise
OS Version: 21H2 19044.2364
API: OpenGL
Occurs on non-Intel GPUs?: No

Steps to Reproduce:
-------------------------------
- Launch attached reproducer executable OR replay attached OpenGL trace using qapitrace (https://github.com/apitrace/apitrace)

Expected Result:
-------------------------------
- Rendering contains colors as seen in OK.png

Actual Result:
-------------------------------
- Rendering is all black as seen in KO.png

Additional Information:
-------------------------------
- The source code of the reproducer is attached (FrontColorIssue.c)

Thanks in advance

[Hugo_Intel]

Hello vsarthou

Thank you for posting on the Intel Communities.

Please allow us more time to check the information that you have provided so far. I will post back once I have an update for you or in case a need more information from you.

Best Regards,

Hugo O.

Intel Customer Support Technician.

[Hugo_Intel]

Hello vsarthou

Thank you for your patience. Please let us know the following information:

• Are you contacting us on behalf of a company for a specific type of project? Or, Are you an independent developer?
• Are you able to test this on and 11th Generation processor or newer?
• Please run Intel® System Support Utility and attach the log file to get more information about your system.

Best Regards,

Hugo O.

Intel Customer Support Technician.

[vsarthou]

Hello Hugo,

Thank you for your reply.

• Yes I am contacting you on behalf of my company for a specific project.
• I was able to test my reproducer on this CPU: 12th Gen Intel(R) Core(TM) i7-12700T
  And the bug is still reproducible.
• You will find attached the report generated by Intel System Support Utility (sys_info.txt)

Regards,

[Hugo_Intel]

Hello vsarthou

Thank you for sharing these details with us. We will continue researching the information you have provided so far and I will post back as soon as I have an update for you.

Best Regards,

Hugo O.

Intel Customer Support Technician.

[Hugo_Intel]

Hello vsarthou

Thank you for your patience, We have determined that in the case of your query, it will be better to route your issue to our developer zone. Please refer to the following link so you can get further support:

Intel®️ Developer Zone - Contact Page   

https://software.intel.com/en-us/support 

Please note that this thread will no longer be monitored. Thank you for your understanding.

Best regards, 

Hugo O.   

Intel Customer Support Technician.